[LAA] Convert test to opaque pointers (NFC)

When converting this test to opaque pointers (and dropping bitcast),
we get improved memory checks. Per fhahn:

> It looks like the difference is due to the logic that determines
> pointer strides in LAA not handling bitcasts. Without the
> bitcasts, the logic now triggers successfully.

Differential Revision: https://reviews.llvm.org/D140204